The present invention relates to an ink composition for waterless lithographic printing of a seamless can for retort food which is excellent in retort resistance and does not have an adverse effect on environment, and a printing method using the same ink composition.
On an outer surface of a seamless can, indication of its contents and necessary instructions are printed. For such printing, letterpress dry offset printing and lithographic printing methods are adopted. The offset printing method is a method of once transferring an image from a printing plate to a blanket and then printing on an article to be printed, and also the lithographic printing method employs an offset and therefore is classified into the offset printing method. In the letterpress dry offset printing method, defacing of printed letters and dotted image easily arises, but since an inker unit thereof is compact and high speed printing can be carried out, this printing method is mainly used currently.
Generally lithographic printing usually uses damping water, but there is a case where bleeding of an image occurs due to mixing of water in the ink. Further there are disadvantages that a large-sized inker unit is required and it is difficult to increase a printing speed. Therefore the lithographic printing method has hardly been adopted in the filed of printing of a seamless can.
Recently in order to compensate for those disadvantages, there is proposed a method of carrying out waterless lithographic printing by mounting a waterless lithographic plate on a conventional letterpress dry offset printer as disclosed in JP2001-129966A, and this method is put into practical use for printing of a seamless can in which retort treatment is not necessary.
In the waterless lithographic printing method, a non-printing portion is made of a silicone rubber having a property of repelling an ink, and the method comprises transferring the ink only to a printing portion made of aluminum, once transferring an image of the printing portion to a blanket and then pressing the blanket onto an outer surface of the seamless can to transfer the ink.
Also an ink to be used for such a waterless lithographic printing method is demanded to have specific properties. Usual inks for lithographic printing and letterpress printing cannot be used as they are for the waterless lithographic printing.
Important properties required for an ink for waterless lithographic printing are resistance to stain of non-printing portion and transferability. When a temperature of a printing plate is increased during printing, an ink is lowered in its aggregation to adhere to a non-printing portion. This phenomenon is so-called as stain of non-printing portion. The resistance to stain of non-printing portion is a property to prevent the phenomenon.
In order to prevent this stain of non-printing portion, a method of increasing an aggregation force of a binder resin of the ink (increase in a molecular weight and concentration of a resin, etc.) and a method of decreasing an adhering force of the ink on a non-printing portion (addition of silicone oil, etc.) are studied.
However in the case of increasing a molecular weight of a resin, selection of a solvent is difficult, and though the increase in a molecular weight of a resin has an effect on reduction of ink adhesion on a non-printing portion, transferability of ink to a printing portion is lowered.
For waterless lithographic printing, a rosin-modified phenol resin is most generally used as an ink binder resin, and when a molecular weight of this resin is made high, an aromatic hydrocarbon solvent becomes essential as a solvent.
On the other hand, it is desired not to use an aromatic hydrocarbon solvent in consideration of an effect on environment. Also for an ink for lithographic printing, solvent systems using no aromatic hydrocarbon solvent are proposed.
For example, in JP5-247394A, there is proposed a solvent mixture comprising an alkyl-substituted aromatic hydrocarbon solvent and a solvent comprising naphthene and paraffin as main components. In JP-5-247395, a petroleum solvent containing a n-paraffin component in an amount of not less than 90% by weight is proposed.
JP10-292145A proposes that a liquid polyorganosiloxane is mixed for enhancement of transferability and also a content of an aromatic hydrocarbon solvent is adjusted to be not more than 1% by weight.
Also in JP2003-12988A, there is proposed a solvent which contains diaryl alkane having 15 to 18 carbon atoms in an amount of not less than 10% by weight and has an aniline point of not more than 90° C. and an initial boiling point of not less than 290° C. This patent publication says that an aromatic hydrocarbon solvent, aliphatic hydrocarbon solvent and general solvents such as alcohol solvent and ether solvent can be used as other solvent.
However in JP5-247394A and JP2003-12988A, a solvent is an alkyl-modified aromatic hydrocarbon or aryl-modified alkane, and both of them are solvents comprising aromatic hydrocarbon as a basic component, and an effect on environment cannot be estimated.
JP5-247395A discloses an effect of using n-paraffin, but use of an aromatic hydrocarbon solvent is allowed and an effect on environment cannot be estimated.
JP10-292145A says that transferability cannot be enhanced unless a liquid polyorganosiloxane is mixed, but it is known that the polyorganosiloxane repels a varnish to be coated on an ink and has an adverse effect on a surface condition.
Moreover any of those patent publications, JP5-247394A, JP-5-247395, JP10-292145A and JP2003-12988A are improvements when using a rosin-modified phenol resin as a binder resin, and there is no teaching as to development to other resins.
Also though a method of waterless lithographic printing of a seamless can which uses an alkyd resin as a binder resin is disclosed in JP2002-103775A, there is no disclosure with respect to a solvent.
On the other hand, many seamless cans are used as a vessel for retort foods. For vessels for retort foods, resistance to heating in a boiling water, heating in a microwave oven, etc. is demanded from the viewpoint of applications thereof. Similarly retort resistance is demanded for printing of an outer surface of such cans.
In order to impart retort resistance to an ink for waterless lithographic printing, generally a mechanical strength of a binder resin is increased by crosslinking. For example, curing agents comprising an amine resin such as a methylolmelamine resin are used as a curing agent. However since an amine resin curing agent itself has weak repellency to a silicone rubber of a non-printing portion, there is a case where not only staining of a non-printing portion but also generation of formaldehyde which is said to have an effect on environment occurs. Therefore it is desired not to use such a curing agent.